then gradually come down, and when it is within five or six feet
of the ground the man must instantly strike the wings downwards,
and sit as far back as he can; he will by this means check the
projectile force, and cause the car to alight very gently with a
retrograde motion. The car, when up in the air, may be made to
turn to the right or to the left by forcing out one of the fins,
having one about eighteen inches long placed vertically on each
side of the car for that purpose, or perhaps merely by the man
inclining the weight of his body to one side.'

Having stated how the thing is to be done, Walker is careful to
explain that when it is done there will be in it some practical
use, notably in respect of the conveyance of mails and
newspapers, or the saving of life at sea, or for exploration,
etc. It might even reduce the number of horses kept by man for
his use, by means of which a large amount of land might be set
free for the growth of food for human consumption.

At the end of his work Walker admits the idea of steam power for
driving a flying machine in place of simple human exertion, but
he, like Cayley, saw a drawback to this in the weight of the
necessary engine. On the whole, he concluded, navigation of the
air by means of engine power would be mostly confined to the
construction of navigable balloons.

As already noted, Walker's work is not over practical, and the
foregoing extract includes the most practical part of it; the
rest is a series of dissertations on bird flight, in which,
evidently, the portrait painter's observations were far less
thorough than those of da Vinci or Borelli. Taken on the whole,